So-called medium-refractive-index low-dispersion optical lenses, which have a refractive index nd of 1.55-1.65 and an Abbe number vd of 55-65, are in wide use in the optical pickups of various optical disk systems including a CD, DVD, or the like and in video cameras, digital cameras, etc. The demand for these articles is growing remarkably in recent years and, with this trend, there is a desire for a medium-refractive-index low-dispersion optical lens which is more inexpensive and has high productivity.
For producing such optical lenses, the mold press forming process is extensively used in recent years as a process which attains high productivity and a low production cost. In this production process, a preformed glass heated to a temperature not lower than the glass transition temperature is pressed with two pairs of forming molds having high-precision surfaces and disposed on the upper and lower sides to thereby realize an optical lens having a desired shape. Compared to the conventional optical-lens production based on grinding/polishing, the mold press forming process comprises a smaller number of production steps and hence attains a lower production cost. There is a desire for a glass material capable of production by this process.
In order for an optical lens to be inexpensively produced by the mold press process, the glass is desired to have a low yield point Ts. This is because although molds having a release film or the like on the surface thereof are generally used in the mold press process in order to prevent the molds from fusion-bonding to the heated glass, the release film deteriorates upon exposure to high temperatures. Namely, for inexpensively producing an optical lens by the mold press process, an optical glass having a low yield point and low-temperature-softening properties is necessary.
An SiO2—PbO—R2O glass has hitherto been used as an optical glass which has a medium refractive index and low-dispersion characteristics and has low-temperature-softening properties suitable for mold press forming. However, from the standpoint of concern for the environment, lead-free optical glasses are being developed recently. For example, an optical glass having low-temperature-softening properties, a medium refractive index, and low-dispersion characteristics has been proposed which is obtained by adding an alkali oxide such as Li2O, Na2O, or K2O to an SiO2—B2O3—RO glass.
However, it is known that addition of an alkali oxide to an SiO2—B2O—RO glass generally results in impaired weatherability. Namely, low-temperature-softening properties and weatherability are antinomic. For example, the optical glass described in patent document 1 is sufficiently low in yield point and suitable for mold pressing but has insufficient weatherability. This optical glass is desired to be further improved. On the other hand, the optical glasses described in patent documents 2 and 3 have sufficient weatherability but have a relatively high yield point. These optical glasses are desired to have a lower yield point so as to produce an optical lens at a lower cost.
Patent Document 1: JP-A-7-149536
Patent Document 2: JP-A-60-122747
Patent Document 3: JP-A-4-37628